Sharks, skates, and rays, which together form a group of about 900-1150 species(1,2) of ocean-dwelling and freshwater-dwelling fish(3,4) called elasmobranchs,(1) are some of the most fascinating creatures of the deep. While they come in many sizes and shapes—from the giant whale shark(5) and the huge manta ray (3) to the dwarf lanternshark(6) and the tiny short-nosed electric ray,(3) and the from the odd-looking hammerhead sharks(4) to the totally bizarre sawfish(3)—all living elasmobranchs share certain key features. First of all, their skeletons are made up of a strong, flexible, and light material called cartilage,(7) rather than bone, making them (along with another fish group called chimaeras(1)) fundamentally different from other fish.(1,3,8) Other important characteristics include their rows of replaceable teeth(2) and the 5-7 gill slits on each side of their body.(3) In addition, although these creatures are ancient—the first elasmobranchs evolved at least 400 million years ago!(1,5,8,9)—they have many highly-developed senses,(4,10) including the amazing ability to perceive tiny changes in electricity around them.(10,11) Sharks, as well as rays and skates (which you can tell apart from sharks by their generally flattened, diamond-shaped bodies(3)), often use this sense for finding prey, as well as for finding their way through the water.(10,11) In at least some elasmobranchs this sense may even be used in various social and mating behaviors.(11) In part thanks to this electric sense, many elasmobranchs are skillful hunters, often serving as the top predators in the food chain and keeping their environments in the proper balance.(9,12,13) These creatures also have an important relationship with humans. Some rays, such as stingrays and electric rays, can cause injury to people.(3,4) And even if you haven’t heard much about the dangers of those fish, you’ve definitely heard of shark attacks, which have given sharks a very dark reputation even though these attacks are actually rare.(4,12,14) In fact, although elasmobranchs can pose dangers to humans, humans pose a much greater danger to them.(13) For over 5000 years shark meat has been eaten by people,(8) and ray meat, skate meat, shark skin, and other elasmobranch products are also sometimes used by humans today.(3,8,13) Overfishing, accidental catching (called “bycatch”),(1,9,13,15,16) higher numbers of people living on the coast, and greater damage to coastal environments(13,15) are all threatening sharks, skates, and rays. And because elasmobranchs generally grow slowly, reproduce late in life, and have only a small number of children, they have trouble recovering from population decline caused by humans.(1,5,8,12,13,15) As a result, many elasmobranchs around the world are endangered.(13,15)
Description of Elasmobranchii
From the enormous whale sharks(1) and manta rays(2) to the tiny dwarf lanternsharks(3) and short-nosed electric rays,(2) the elasmobranchs (the sharks, skates, and rays(4)) are a diverse group of fishes with some 900-1150 species(4,5) living all over the world (6,7) in both marine and freshwater habitats.(2,8) The elasmobranchs and one other group, the chimaeras, make up the class of cartilaginous fish or chondrichthyans;(4) as these fish have skeletons made up of a strong, flexible, and light material called cartilage,(9) rather than bone, they are fundamentally different from other fish.(2,4,7) Living elasmobranchs also share, among other physical features, rows of replaceable teeth(5) and 5-7 gill slits on each side of their body.(2) Although these creatures are ancient—the first elasmobranchs evolved at least 400 million years ago(1,4,6,7)—they have many sophisticated senses,(8,10) including the ability to perceive very small shifts in electricity around them.(10,11) Sharks, as well as rays and skates (distinguishable from sharks by, among other characteristics, their generally flattened, diamond-shaped bodies(2)), frequently use this sense for finding prey, as well as for navigation;(10,11) in at least some elasmobranchs this sense may even be connected with various social and mating behaviors.(11) As the skillful hunters that many of them are, elasmobranchs are crucial parts of their environments, often serving as the top predators in the food chain and keeping the ecosystem in balance.(6,12,13) These animals also have an important relationship with humans. Some rays, such as stingrays and electric rays, can cause injury to people.(2,8) Much more present in the public imagination are shark attacks, which, while in fact rare, have given sharks a very dark reputation.(8,12,14) But although elasmobranchs can pose dangers to humans, humans pose a much greater danger to them.(13) For over 5000 years shark meat has been used as a food source and in some countries today it is considered a delicacy;(7) ray and skate meat are also sometimes consumed (2) and shark skin is an expensive product in some markets.(7) Because elasmobranchs generally grow slowly, reproduce late in life, and produce only a small number of offspring, they are particularly sensitive to human exploitation,(1,4,7,12,13,15) and as a result of overfishing and accidental catching (“bycatch”)(4,6,13,15,16)—as well as increasing human presence on and damage to the coastal environment(13,15)—many elasmobranchs worldwide are in decline.(13)
Known prey organisms
Based on studies in:
South Africa (Desert or dune)
This list may not be complete but is based on published studies.
Life History and Behavior
As an apex predator, sharks play a pivotal role in controlling the populations of various species of fish and other aquatic life forms. Sharks are not much of a threat towards humans, often mistaking limbs for helpless prey. Species such as the Tiger and Bull will attempt to eat anything, more so due to the sake of their sizes. Sharks are carnivorous, but they very rarely change up their diet; most species will eat plankton and smaller types of fish.
Sharks are very migratory, often spanning vast amounts of ocean to meet at breeding grounds. They will swim in schools, and are very social amongst one another. Their level of intelligence is quite high, and, for example, show signs of curiosity when playing and will move erratically when they feel threatened.
Evolution and Systematics
Sharks are efficient swimmers in part due to their complex hydroskeleton.
"It came as more of a surprise than it should have when Wainwright, Vosburgh, and Hebrank (1978) showed that sharks utilized a hydroskeleton. Sharks, after all, have conventional skeletons, albeit somewhat less calcified ones than that of most other vertebrates. Shark skin is sturdier stuff than fish skins generally…and it has the crossed helical fiber array typical of these hydrostatic arrangements, shown in figure 20.5b. The system, though, is more complex than those described previously--muscles attach directly to the skin, which thus acts both as external, pressure-resisting membrane and as an external, whole-body tendon. Fiber angles, not unreasonably, vary with location on the fish--the unstretchy skin must transmit the forces generated by the body musculature back toward the tail. During locomotion, the pressure inside the body of a shark rises to as much as 200 kilopascals--twice atmospheric and as high as that inside an automobile tire. So sharks are just shark-shaped balloons with teeth.
"In sharks, the peculiar resistance of crossed helical fibers to torsion may have functional significance, at least of a negative kind. Sharks lack swimbladders and thus sink if inactive. Swimming must produce a little lift in addition to thrust, and a shark gets that lift, in part, by beating a tail fin that extends farther dorsally than ventrally, as you can see in the figure. That asymmetry might make a shark uselessly twist lengthwise, reversing twist twice during each full tailbeat--were it not for a torso that, while flexible in bending, resists twisting." (Vogel 2003:415-416)
Learn more about this functional adaptation.
The skin of cartilaginous fish is protected by a covering of abrasive placoid scales, called denticles.
"The other main type of fish scales are those known as placoid scales or, more commonly and appropriately, denticles: 'little teeth' (diagram d). They are found on the primitive cartilaginous fishes, sharks, skates, and rays (whose skeletons are made of cartilage, not bone). Each denticle grows up from the dermis until its curved tip breaks the skin surface -- denticles are not covered with skin as bony scales are. Each denticle, like a human tooth, is made of dentine (tooth ivory) capped with enamel; each has a pulp cavity containing nerves and blood vessels. Denticles are usually small, but may be sharp. Brushing against the skin of a shark, can flay the skin of a swimming man like a particularly vicious sandpaper." (Foy and Oxford Scientific Films 1982:91)
Learn more about this functional adaptation.
Molecular Biology and Genetics
Statistics of barcoding coverage
Specimens with Sequences:16537
Specimens with Barcodes:11900
Species With Barcodes:987
EOL content is automatically assembled from many different content providers. As a result, from time to time you may find pages on EOL that are confusing.
To request an improvement, please leave a comment on the page. Thank you!